It's probably a lack of imagination on the part of microphone designers, but you have to wonder why virtually all microphone diaphragms are circular.
I guess the circle seems a kind of obvious shape, something you couldn't really go wrong with. But might there be advantages in having a diaphragm of some other shape?
Pearl Microphone Laboratory things so. Take a look at their ELM-C model – the diaphragm is distinctly rectangular, in fact the length of the diaphragm is a full seven times the width!
Pearl claim that the problem with the circular diaphragm is that its primary resonance is highly pronounced. All microphone diaphragms have a certain resonant frequency, and this will tend to emphasize a band of frequencies and generally mess it about in terms of level and smear it over time.
But a rectangular diaphragm will have a certain resonant frequency due to its width, and another due to its length. The resonance due to the long dimension is apparently weakened, resulting in an overall lowering in the degree of resonance of the diaphragm. Of course a resonance can always be damped, but it is better if it is not too strong in the first place.
The other interesting point about a rectangular diaphragm is that it has a different directional pattern in the horizontal plane to that in the vertical plane. If the microphone is orientated vertically, then it will have a wide horizontal directional pattern but a very narrow vertical pattern.
So if you wanted to reduce pickup of reflections from the ceiling and floor, this would be the mic for you. Also, if it turned out that it was the reflections from the walls that were the problem, simply mount the mic horizontally.
It would be interesting to try out this mic for vocals since a vertical positioning would focus tightly on the mouth, whereas a horizontal positioning would pick up the head and chest resonances too. Another possible application would be drum miking, where directionality is important.
Of course the next step would be for manufacturers to develop all sorts of diaphragm shapes. And who says they have to be flat?
The world needs more interesting microphones!